The present invention relates to a fuel supplying device for use in fuel injection type internal combustion engine and, more particularly, to a fuel supplying device of the kind provided with an air valve disposed in an air horn upstream of a throttle valve. This air valve is provided for maintaining the intake pressure in the air horn at a predetermined level relative to the ambient atmospheric pressure so that the flow quantity of air sucked through the air horn may be in proportion to the opening degree of the air valve.
In the fuel supplying device of the kind described, the fuel is metered by means of a fuel metering device which is operatively connected to the air valve. This fuel metering device incorporates a metering orifice the opening degree of which is controlled in accordance with the opening degree of the air valve. The fuel is fed from a fuel tank to the fuel metering device, by means of a fuel pump, and is metered in accordance with the opening degree of the metering orifice incorporated in the fuel metering device. Thus, the fuel is metered in proportion to the opening degree of the metering orifice, provided that the pressure difference across the fuel metering device, particularly across the metering orifice, is maintained constant.
Namely, since the opening degree of the fuel metering device is in proportion to that of the air valve, the fuel is metered in proportion to the opening degree of the air valve.
A constant differential pressure valve is provided for maintaining a predetermined differential pressure across the fuel metering device. The constant differential pressure valve of a conventional fuel supplying device is arranged to variably restrict the fuel downstream of the fuel metering device, i.e. the metered fuel, in response to the differential pressure across the fuel metering device so as to control the pressure at the outlet of the fuel metering device, thereby to maintain the predetermined differential pressure.
This conventional fuel supplying device, however, has the following disadvantage. Namely, since the fuel metering device is of the type wherein the metered fuel to be supplied to the engine is totally restricted in response to the differential pressure, the conventional constant differential pressure valve should have a relatively large capacity. Usually, this constant differential pressure valve has a diaphragm sensitive to the differential pressure and a nozzle opposing to the diaphragm, between which a clearance is defined so as to restrict the metered fuel. The clearance is varied through the change in the deflection of the diaphragm in response to the change in the differential pressure. The metered fuel flows through this nozzle to an injector, and is discharged from the latter into the air horn.
The orifice diameter of the nozzle must have a increased dimension enough to allow the maximum flow quantity of the metered fuel consumed by the engine. Thus, due to the increased orifice diameter of the nozzle, the precise restriction control of the metered fuel becomes difficult through the change in the clearance between the diaphragm and the nozzle. Since the maximum clearance between the nozzle and the diaphragm should allow the aforementioned maximum flow quantity of the metered fuel, the clearance should change over a wide range in association with the change in the flow quantity of the metered fuel from the minimum to the maximum. This in turn causes an increased lift of the diaphragm between the minimum and maximum clearance positions. In order to have the diaphragm smoothly deflected over a increased lift, the diaphragm has to have a sufficiently large diameter.
The increased diameter of the diaphragm inconveniently results in delay and disturbance in the transient response thereof, although it can improve the accuracy of the control. At the same time, an excessively large lift of the diaphragm may cause a permanent strain of the diaphragm resulting in a deterioration of the precise control and in a lack of its reliability.
To minimize the required capacity of the constant differential pressure valve, it has been proposed to provide a plurality of the fuel metering devices each including the constant differential pressure valve, preferably corresponding to the number of the cylinders of an engine. However, this proposition is not practical, because the increased number of component parts causes the high cost of an engine.